Currently, robotic manipulators are most often actuated by position control of joints that possess unmodulated stiffness. This limits most mechanical manipulators to mainly non-contact operations. On the other hand, human body by means of fingers or hands is able to manipulate objects and to master the object's dynamic response and thereby, interact well with the environment. Robotic manipulators could thus benefit from the teachings of human anatomy which suggests that both independent modulation of stiffness and position is needed in order to execute constrained maneuvers.
Referring to prior art FIG. 1(a), a typical human joint is shown. Simplistically, a typical human joint is made up of two major bones which move at some angle relative to each other by means of abductor muscles and flexor muscles. The abductor and the flexor muscles form an antagonistic actuation scheme in which the abductor and the flexor are being activated to determine two joint outputs, namely, the joint position or angle 8 and the joint stiffness K. These two outputs are being controlled via two separate inputs.
Actuator and kinematic redundancies are the means by which the human musco-skeletal system controls its dynamic response. Human muscles are activated by neural commands to maintain a desired stiffness. The activation of coantagonistic muscles modulates the stiffness of a skeletal joint, while maintaining its angular position. Decoupling the stiffness modulation from the angular positioning is an important characteristic that enables the musco-skeletal system to master grasping and manipulating objects.
Referring to prior art FIG. 1(b), a schematic is shown which relates how the interaction of the abductor and flexor muscles produces the angular .THETA. and stiffness K outputs. The activation input or effort to move the abductor muscle E.sub.ab minus the effort to move the flexor muscle E.sub.f1 relates to the angular output .THETA.. In other words, the difference between the activation levels of the abductor and the flexor determine the variations in the joint position.
The effort to move the abductor muscle E.sub.ab plus the effort to move the flexor muscle E.sub.f1 relates to the stiffness output K. In other words, the sum of these two activations, abductor and flexor, determine the stiffness of the joint and these two control inputs are decoupled in the sense that the difference modulates the position versus the sum which modulates the stiffness.
Note that an important characteristic of a human joint is that there is no coupling between the activation inputs, i.e., E.sub.ab -E.sub.f1 and E.sub.ab +E.sub.f1, and the outputs, i.e., .THETA. and K, as shown by the schematic in FIG. 1(b). Like the human musco-skeletal system, robot-environment interactions can benefit by being controlled through the stiffness modulation of its joints.
Referring to FIG. 2, a schematic of a typical robot joint actuator is shown. This type of actuation scheme is often used in assembly work. Robots use basically only one input namely, position control of angles to produce an angular output .THETA.. The stiffness output is constant as the stiffness does not vary but is instead built into the machine. Such an actuation scheme does not tend to interact very well with a changing environment and has only limited applications.
Referring to prior art FIG. 3, a schematic of a compliance control actuation scheme currently being used is shown. In this actuation scheme, an active control strategy is used. Active control is really related to closed loop control strategy in which the position .THETA. of the joint is measured and compared with its desired position .THETA..sub.d to calculate the error in the position. This error is translated into an applied torque T which is a function of the desired stiffness K introduced to the joint. Thus, the stiffness times the error in positioning results in an applied torque T on that joint. The closing of the loop creates reliability and stability problems that makes this actuation scheme only workable in a laboratory environment and little used in industry.
In robotic operations that expose the robotic manipulator to physical contact with the environment, small positioning errors result in large interacting forces. These large forces are due to the high stiffness that the mechanical manipulator possess. High manipulator stiffness is required for positioning accuracy. Hence, high positioning accuracy and low contact forces are conflicting requirements for conventional robotic systems. This conflict has limited the robotic applications that expose the manipulator to physical contact with its environment.
Mechanical manipulators that are position controlled have had a limited success in physical contact operations. The advantages of compliant motion control have been recognized, yet industrial compliance control manipulators do not exist. The present invention has been developed to control the angular position of a joint and independently modulate its stiffness. This actuator enables the automation of contact operations that position controlled robots fail to execute.
The importance of compliant motion control has been recognized, however, commercial compliance controlled manipulators do not exist. Most robotic manipulators emphasize position control, while overlooking compliance control schemes. Recently, however, research has contemplated compliant motion control schemes as a remedy. For instance, research has been conducted by N. Hogan to experiment with modulating end-effector compliance by utilizing actuator redundancies.
Other research by D. E. Whitney, for example, has introduced active control strategies that utilize force sensors to monitor physical interactions and determine appropriate motion commands. In this scheme, force sensors are being utilized to monitor the physical interactions and determine appropriate motion commands. What the force sensors really are monitoring is the physical interaction between the manipulator and its environment.
Referring to prior art FIG. 4, other research by J. K. Mills studied schemes of coantagonistic bladder actuators, for which the stiffness modulation and angular positioning are highly coupled. This undesired coupling may be eliminated only through complex control algorithms.
It is an object of the present invention to develop a new non-antagonistic actuation scheme, that decouples the stiffness modulation from its angular positioning to drive a manipulator, while at the same time modulating the manipulator's end-point stiffness.
It is a further object of the present invention to develop an angular positioning and stiffness modulating actuator of a small enough size and weight to be placed at a joint in order to be commercially useful.
It is a further object of the present invention to develop an angular positioning and stiffness modulating actuator which accomplishes both position control and stiffness control in open loop to make the actuation scheme more stable and reliable and less expensive than existing compliant control schemes.
It is a further object of the present invention to develop an angular positioning and stiffness modulating actuator which simplifies present compliance control schemes in that no sensing and feedback functions are necessary.